1. Field of the Invention
The present invention relates to a developing device used for the image development in an image forming apparatus such as a copier, a composite apparatus, or a printer, and an image forming apparatus using the same.
2. Description of the Background Art
A developing device 900 used upon developing a latent image formed on a photosensitive drum of an image forming apparatus into a visible image is disposed at a position proximate to a photosensitive drum 901 as shown in FIG. 9. The developing device 900 is provided with a housing 902 holding toner, a developing roller 903 supported on the housing 902, and conveying screws 904, 905. Blades 906 having a spiral shape and a uniform diameter for agitating and circulating the toner in the housing 902 are provided on rotary shafts 907 of the conveying screws 904, 905 (see Japanese Unexamined Patent Publication No. 2001-255723).
As shown in FIG. 10, the developing roller 903 is constructed such that the outer circumferential surface of a magnet roller 908 is covered by a developing sleeve 909. The magnetic roller 908 has a five-pole structure. The magnetic roller 908 is fixed, whereas the developing sleeve 909 is rotatable. The toner in the housing 902 agitated and circulated by the conveying screws 904, 905 is conveyed to the outside of the housing 902 by the rotation of the developing roller 903.
A charger 910 for charging the toner conveyed by the developing roller 903 is disposed above the developing roller 903 in the housing 902. The toner conveyed to the outside by the developing roller 903 is supplied to the photosensitive drum 901 while having specified electric charges given thereto by this charger 910 and having an amount thereof restricted to a specified amount.
The outer circumferential surface of the photosensitive drum 901 is uniformly charged, and a latent image is thereafter formed thereon by a laser. A charged state at a position where the latent image was formed in this way is such that the charged toner supplied from the developing roller 903 is attached thereto. Thus, the toner supplied from the developing roller 903 attaches only at the position where the latent image was formed.
Out of the toner supplied to the photosensitive drum 901, the one that has not been attached to the photosensitive drum 901 is collected into the housing 902 by remaining on the developing roller 903. When the toner that has not been attached to the photosensitive drum 901 is collected into the housing 902, the charged toner is present in the housing 902. If the thus collected toner is supplied again to the photosensitive drum 910 by the developing roller 903, electric charges are given thereto by the charger 901 upon the supply to the photosensitive drum 901 despite the fact that the toner is already charged. Since the toner is excessively charged if electric charges are given thereto again, there occurs a development error such as the attachment of the toner to a part of the outer circumferential surface of the photosensitive drum 901 where no latent image is formed.
A development error as above occurs also when the collected toner is locally present in part of the interior of the housing 902 instead of being uniformly present in the housing 902, and the locally present collected toner is supplied to the photosensitive drum 901 by the developing roller 903.
In order to avoid such a development error, in the developing device 900, the toner is agitated by the conveying screws 904, 905 to uniformly disperse the charged toner in the housing 902.
However, with the conveying screws provided with the spiral blades having a uniform diameter disclosed in Japanese Unexamined Patent Publication No. 2001-255723, the flow rate of the toner circulating in the housing is constant and the toner cannot be sufficiently agitated since a conveying ability is uniform at any position along the axial direction of the rotary shafts of the conveying screws.
Further, if the toner is pressed more by the conveying screws in order to improve the agitation efficiency, an externally added agent (e.g. abrasive) is buried in the outer surfaces of toner particles or come off from the outer surfaces of toner particles due to resulting stresses, wherefore a toner conveyance error occurs due to the excessive charging of the toner and a reduction in the fluidity of the toner.